1. Field of the Invention
The present invention relates to a ceramic honeycomb structure used as a catalyst carrier for automobile exhaust gas purifiers and the like, and more particularly it relates to a ceramic honeycomb structure with excellent thermal shock resistance.
2. Description of the Related Art
As shown in FIGS. 6A and 6B, conventional catalyst carriers for exhaust gas purifiers have a ceramic honeycomb structure 9 wherein a plurality of cells 99 are formed by partitions 90, made of a ceramic such as cordierite, which are arranged in a lattice, and surrounded at the perimeter by an outer shell 95 made of the same material as the partitions 90. The exhaust gas purification catalyst is held on the surface of the partitions 90 of the ceramic honeycomb structure 9 whereby the exhaust gas purifying function is exhibited.
When this type of ceramic honeycomb structure 9 is used as a catalyst carrier in an exhaust gas purifier, it is directly exposed to high-temperature exhaust gas and is subjected to repeated thermal shocks. Such thermal shocks often lead to cracking in the outer shell or other parts of the ceramic honeycomb structure. The ceramic honeycomb structure must therefore exhibit high thermal shock resistance so that the thermal shocks received during use do not cause cracking.
Also, with the recent demand for high performance from exhaust gas purifiers, modifications are being made to allow a more rapid temperature increase than in conventional catalyst carriers, so that the catalytic function can be exhibited earlier. For example, the partitions may be made thinner in order to lower the heat capacity of the partitions. When the partitions are made thinner, the outer shell is thickened in order to provide the required strength for the entire ceramic honeycomb structure. Such thinning of the partitions and thickening of the outer shell results in greater stress on the outer shell upon thermal shock, thus rendering it more susceptible to cracking.
For this reason it has been desired to further improve the thermal shock resistance of ceramic honeycomb structures.
It is an object of the present invention, which has been accomplished in light of these problems of the prior art, to provide a ceramic honeycomb structure with excellent thermal shock resistance.
The present invention provides a ceramic honeycomb structure comprising partitions arranged in a lattice forming a plurality of cells and an outer shell connected with the partitions and surrounding the perimeter of the partitions,
the ceramic honeycomb structure being characterized in that the outer shell is provided with slits formed by cutting the outer shell in the lengthwise direction along its entire length.
The most notable feature of the invention is the formation of slits in the outer shell along its entire length.
The slits are formed by cutting from the outer shell. The presence of gaps between both edges of the cut slits is optional. That is, the slits may be formed in such a manner that the outer shell portions bordering both sides of each slit are separated and the outer shell portions on both sides may thus be separated by a gap preventing their contact, or they maybe in contact without a gap.
The method of forming the slits is preferably a method in which they are formed simultaneously with extrusion molding of the ceramic honeycomb structure. This allows the slits to be formed without requiring any special steps, and also allows satisfactory finishing of the slit width, shape, etc.
The partitions and outer shell are constructed of a ceramic such as, for example, cordierite. Cordierite has the general chemical composition of SiO2: 45-55 wt %, Al2O3: 33-42 wt %, MgO: 12-18 wt %.
As mentioned above, the ceramic honeycomb structure of the invention has slits in the outer shell. The ceramic honeycomb structure therefore has thermal shock resistance which is vastly superior to that of a structure in which no slits are provided.
When the ceramic honeycomb structure is suddenly heated by exhaust gas, for example, it undergoes a sudden thermal expansion. The thermal expansion is usually greatest in the partition sections through which most of the exhaust gas passes, and the thermal expansion is different from that of the outer Shell. A large tensile stress is applied to the outer shell as a result. Consequently, cracks have often occurred in conventional outer shells that cannot withstand such tensile stress.
According to the invention, the aforementioned slits are provided in the outer shell, thus dividing the outer shell. As a result, when tensile stress is applied to the outer shell, the widths of the slits become larger so that the tensile stress is adequately released. Moreover, since the slits are provided along the entire length of the outer shell, it is also possible to avoid a concentration of stress. A ceramic honeycomb structure of the invention with such slits therefore exhibits vastly improved thermal shock resistance over the prior art.
Thus, according to the invention, it is possible to provide a ceramic honeycomb structure with excellent thermal shock resistance.
The cut cross-sections in the outer shell formed by the slits are preferably in contact with each other. That is, the slit width is preferably zero. This eliminates any gaps in the slit sections, thus preventing outer shell chipping and the like.
A plurality of the slits are preferably formed. This will allow thermal shock stress release at a plurality of locations, and thereby improve the thermal shock resistance.
The slits are preferably formed with roughly equivalent spacing between them. This will allow balanced stress release upon subjection to thermal shock, thus improving the thermal shock resistance.
The spacings between the slits are preferably provided such that at least two partitions are connected to the outer shell between adjacent slits. If the spacing between the slits is provided such that less than two partitions are connected to the outer shell between adjacent slits, it may not be possible to achieve an effect of reinforcement of the honeycomb structure with the outer shell.
The outer shell preferably has a cylindrical shape with an outer diameter of 50-150 mm and a length of 50-200 mm, the thickness of the partitions is preferably 0.04-0.3 mm, and the cells are preferably square or hexagonal with a length on one side of 0.5-1.5 mm. A ceramic honeycomb structure of such dimensions exerts stress on the outer shell particularly upon thermal shock, but the slits can reliably provide stress release and thus realize excellent thermal shock resistance.